r"""
Weyl Groups

REFERENCES:

.. [Dye] Dyer. *Bruhat intervals, polyhedral cones and Kazhdan-Lusztig-Stanley polynomials*. Math.Z., 215(2):223-236, 1994.
.. [JahStu] Jahn and Stump. *Bruhat intervals, subword complexes and brick polyhedra for finite Coxeter groups*. Preprint, available at :arxiv:`2103.03715`, 2021.
"""
#*****************************************************************************
#  Copyright (C) 2009    Nicolas M. Thiery <nthiery at users.sf.net>
#
#  Distributed under the terms of the GNU General Public License (GPL)
#                  http://www.gnu.org/licenses/
#******************************************************************************

from sage.misc.cachefunc import cached_method, cached_in_parent_method
from sage.misc.lazy_import import LazyImport
from sage.categories.category_singleton import Category_singleton
from sage.categories.coxeter_groups import CoxeterGroups


class WeylGroups(Category_singleton):
    r"""
    The category of Weyl groups

    See the :wikipedia:`Wikipedia page of Weyl Groups <Weyl_group>`.

    EXAMPLES::

        sage: WeylGroups()
        Category of weyl groups
        sage: WeylGroups().super_categories()
        [Category of coxeter groups]

    Here are some examples::

        sage: WeylGroups().example()            # todo: not implemented
        sage: FiniteWeylGroups().example()
        The symmetric group on {0, ..., 3}
        sage: AffineWeylGroups().example()      # todo: not implemented
        sage: WeylGroup(["B", 3])
        Weyl Group of type ['B', 3] (as a matrix group acting on the ambient space)

    This one will eventually be also in this category::

        sage: SymmetricGroup(4)
        Symmetric group of order 4! as a permutation group

    TESTS::

        sage: C = WeylGroups()
        sage: TestSuite(C).run()
    """

    def super_categories(self):
        r"""
        EXAMPLES::

            sage: WeylGroups().super_categories()
            [Category of coxeter groups]
        """
        return [CoxeterGroups()]

    def additional_structure(self):
        r"""
        Return  ``None``.

        Indeed, the category of Weyl groups defines no additional
        structure: Weyl groups are a special class of Coxeter groups.

        .. SEEALSO:: :meth:`Category.additional_structure`

        .. TODO:: Should this category be a :class:`CategoryWithAxiom`?

        EXAMPLES::

            sage: WeylGroups().additional_structure()
        """
        return None

    Finite = LazyImport('sage.categories.finite_weyl_groups', 'FiniteWeylGroups')

    class ParentMethods:
        def coxeter_matrix(self):
            """
            Return the Coxeter matrix associated to ``self``.

            EXAMPLES::

                sage: G = WeylGroup(['A',3])
                sage: G.coxeter_matrix()
                [1 3 2]
                [3 1 3]
                [2 3 1]
            """
            return self.cartan_type().coxeter_matrix()

        def pieri_factors(self, *args, **keywords):
            r"""
            Returns the set of Pieri factors in this Weyl group.

            For any type, the set of Pieri factors forms a lower ideal
            in Bruhat order, generated by all the conjugates of some
            special element of the Weyl group. In type `A_n`, this
            special element is `s_n\cdots s_1`, and the conjugates are
            obtained by rotating around this reduced word.

            These are used to compute Stanley symmetric functions.

            .. SEEALSO::

                * :meth:`WeylGroups.ElementMethods.stanley_symmetric_function`
                * :mod:`sage.combinat.root_system.pieri_factors`

            EXAMPLES::

                sage: W = WeylGroup(['A',5,1])
                sage: PF = W.pieri_factors()
                sage: PF.cardinality()
                63

                sage: W = WeylGroup(['B',3])
                sage: PF = W.pieri_factors()
                sage: sorted([w.reduced_word() for w in PF])
                [[],
                 [1],
                 [1, 2],
                 [1, 2, 1],
                 [1, 2, 3],
                 [1, 2, 3, 1],
                 [1, 2, 3, 2],
                 [1, 2, 3, 2, 1],
                 [2],
                 [2, 1],
                 [2, 3],
                 [2, 3, 1],
                 [2, 3, 2],
                 [2, 3, 2, 1],
                 [3],
                 [3, 1],
                 [3, 1, 2],
                 [3, 1, 2, 1],
                 [3, 2],
                 [3, 2, 1]]
                sage: W = WeylGroup(['C',4,1])
                sage: PF = W.pieri_factors()
                sage: W.from_reduced_word([3,2,0]) in PF
                True
            """
            # Do not remove this line which makes sure the pieri factor
            # code is properly inserted inside the Cartan Types
            import sage.combinat.root_system.pieri_factors
            assert sage.combinat.root_system.pieri_factors
            ct = self.cartan_type()
            if hasattr(ct, "PieriFactors"):
                return ct.PieriFactors(self, *args, **keywords)
            raise NotImplementedError("Pieri factors for type {}".format(ct))
        
        def bruhat_cone(self, x, y, side='upper', backend='cdd'):
            r"""
            Return the (upper or lower) Bruhat cone associated to the interval ``[x,y]``.

            To a cover relation `v \prec w` in strong Bruhat order you can assign a positive
            root `\beta` given by the unique reflection `s_\beta` such that `s_\beta v = w`.

            The upper Bruhat cone of the interval `[x,y]` is the non-empty, polyhedral cone generated
            by the roots corresponding to `x \prec a` for all atoms `a` in the interval.
            The lower Bruhat cone of the interval `[x,y]` is the non-empty, polyhedral cone generated
            by the roots corresponding to `c \prec y` for all coatoms `c` in the interval.

            INPUT:

            - ``x`` - an element in the group `W`

            - ``y`` - an element in the group `W`

            - ``side`` (default: ``'upper'``) -- must be one of the following:

              * ``'upper'`` - return the upper Bruhat cone of the interval [``x``, ``y``]
              * ``'lower'`` - return the lower Bruhat cone of the interval [``x``, ``y``]

            - ``backend`` -- string (default: ``'cdd'``); the backend to use to create the polyhedron

            EXAMPLES::

                sage: W = WeylGroup(['A',2])
                sage: x = W.from_reduced_word([1])
                sage: y = W.w0
                sage: W.bruhat_cone(x, y)
                A 2-dimensional polyhedron in QQ^3 defined as the convex hull of 1 vertex and 2 rays

                sage: W = WeylGroup(['E',6])
                sage: x = W.one()
                sage: y = W.w0
                sage: W.bruhat_cone(x, y, side='lower')
                A 6-dimensional polyhedron in QQ^8 defined as the convex hull of 1 vertex and 6 rays

            TESTS::

                sage: W = WeylGroup(['A',2])
                sage: x = W.one()
                sage: y = W.w0
                sage: W.bruhat_cone(x, y, side='nonsense')
                Traceback (most recent call last):
                ...
                ValueError: side must be either 'upper' or 'lower'

            REFERENCES:

            - [Dye]_
            - [JahStu]_
            """
            from sage.modules.free_module_element import vector
            if side == 'upper':
                roots = [vector((x * r * x.inverse()).reflection_to_root().to_ambient())
                         for z, r in x.bruhat_upper_covers_reflections()
                         if z.bruhat_le(y)]
            elif side == 'lower':
                roots = [vector((y * r * y.inverse()).reflection_to_root().to_ambient())
                         for z, r in y.bruhat_lower_covers_reflections()
                         if x.bruhat_le(z)]
            else:
                raise ValueError("side must be either 'upper' or 'lower'")

            from sage.geometry.polyhedron.constructor import Polyhedron
            return Polyhedron(vertices=[vector([0] * self.degree())],
                              rays=roots,
                              ambient_dim=self.degree(),
                              backend=backend)

        @cached_method
        def quantum_bruhat_graph(self, index_set=()):
            r"""
            Return the quantum Bruhat graph of the quotient of the Weyl
            group by a parabolic subgroup `W_J`.

            INPUT:

            - ``index_set`` -- (default: ()) a tuple `J` of nodes of
              the Dynkin diagram

            By default, the value for ``index_set`` indicates that the
            subgroup is trivial and the quotient is the full Weyl group.

            EXAMPLES::

                sage: W = WeylGroup(['A',3], prefix="s")
                sage: g = W.quantum_bruhat_graph((1,3))
                sage: g
                Parabolic Quantum Bruhat Graph of Weyl Group of type ['A', 3] (as a matrix group acting on the ambient space) for nodes (1, 3): Digraph on 6 vertices
                sage: g.vertices()
                [s2*s3*s1*s2, s3*s1*s2, s1*s2, s3*s2, s2, 1]
                sage: g.edges()
                [(s2*s3*s1*s2, s2, alpha[2]),
                 (s3*s1*s2, s2*s3*s1*s2, alpha[1] + alpha[2] + alpha[3]),
                 (s3*s1*s2, 1, alpha[2]),
                 (s1*s2, s3*s1*s2, alpha[2] + alpha[3]),
                 (s3*s2, s3*s1*s2, alpha[1] + alpha[2]),
                 (s2, s1*s2, alpha[1] + alpha[2]),
                 (s2, s3*s2, alpha[2] + alpha[3]),
                 (1, s2, alpha[2])]
                sage: W = WeylGroup(['A',3,1], prefix="s")
                sage: g = W.quantum_bruhat_graph()
                Traceback (most recent call last):
                ...
                ValueError: the Cartan type ['A', 3, 1] is not finite
            """
            if not self.cartan_type().is_finite():
                raise ValueError("the Cartan type {} is not finite".format(self.cartan_type()))

            # This is a modified form of quantum_bruhat_successors.
            # It does not do any error checking and also is more efficient
            #   with how it handles memory and checks by using data stored
            #   at this function level rather than recomputing everything.
            lattice = self.cartan_type().root_system().root_lattice()
            NPR = lattice.nonparabolic_positive_roots(index_set)
            NPR_sum = sum(NPR)
            NPR_data = {}
            double_rho = lattice.sum(lattice.positive_roots()) # = 2 * \rho
            for alpha in NPR:
                ref = alpha.associated_reflection()
                alphacheck = alpha.associated_coroot()
                NPR_data[alpha] = [self.from_reduced_word(ref), # the element
                                   len(ref) == double_rho.scalar(alphacheck) - 1, # is_quantum
                                   NPR_sum.scalar(alphacheck)] # the scalar
            # We also create a temporary cache of lengths as they are
            #   relatively expensive to compute and needed frequently
            visited = {}
            todo = {self.one()}
            len_cache = {}
            def length(x):
                if x in len_cache:
                    return len_cache[x]
                len_cache[x] = x.length()
                return len_cache[x]
            while todo:
                x = todo.pop()
                w_length_plus_one = length(x) + 1
                adj = {}
                for alpha in NPR:
                    elt, is_quantum, scalar = NPR_data[alpha]
                    wr = x * elt
                    wrc = wr.coset_representative(index_set)
                    # coset_representative returns wr if nothing gets changed
                    if wrc is wr and length(wrc) == w_length_plus_one:
                        if wrc not in visited:
                            todo.add(wrc)
                        adj[wr] = alpha
                    elif is_quantum and length(wrc) == w_length_plus_one - scalar:
                        if wrc not in visited:
                            todo.add(wrc)
                        adj[wrc] = alpha
                visited[x] = adj

            from sage.graphs.digraph import DiGraph
            return DiGraph(visited,
                           name="Parabolic Quantum Bruhat Graph of %s for nodes %s"%(self, index_set),
                           format="dict_of_dicts",
                           data_structure="static_sparse")

    class ElementMethods:

        def is_pieri_factor(self):
            r"""
            Returns whether ``self`` is a Pieri factor, as used for
            computing Stanley symmetric functions.

            .. SEEALSO::

                * :meth:`stanley_symmetric_function`
                * :meth:`WeylGroups.ParentMethods.pieri_factors`

            EXAMPLES::

                sage: W = WeylGroup(['A',5,1])
                sage: W.from_reduced_word([3,2,5]).is_pieri_factor()
                True
                sage: W.from_reduced_word([3,2,4,5]).is_pieri_factor()
                False

                sage: W = WeylGroup(['C',4,1])
                sage: W.from_reduced_word([0,2,1]).is_pieri_factor()
                True
                sage: W.from_reduced_word([0,2,1,0]).is_pieri_factor()
                False

                sage: W = WeylGroup(['B',3])
                sage: W.from_reduced_word([3,2,3]).is_pieri_factor()
                False
                sage: W.from_reduced_word([2,1,2]).is_pieri_factor()
                True
            """

            return self in self.parent().pieri_factors()


        def left_pieri_factorizations(self, max_length=None):
            r"""
            Returns all factorizations of ``self`` as `uv`, where `u`
            is a Pieri factor and `v` is an element of the Weyl group.

            .. SEEALSO::

                * :meth:`WeylGroups.ParentMethods.pieri_factors`
                * :mod:`sage.combinat.root_system.pieri_factors`

            EXAMPLES:

            If we take `w = w_0` the maximal element of a strict parabolic
            subgroup of type `A_{n_1} \times \cdots \times A_{n_k}`, then the Pieri
            factorizations are in correspondence with all Pieri factors, and
            there are `\prod 2^{n_i}` of them::

                sage: W = WeylGroup(['A', 4, 1])
                sage: W.from_reduced_word([]).left_pieri_factorizations().cardinality()
                1
                sage: W.from_reduced_word([1]).left_pieri_factorizations().cardinality()
                2
                sage: W.from_reduced_word([1,2,1]).left_pieri_factorizations().cardinality()
                4
                sage: W.from_reduced_word([1,2,3,1,2,1]).left_pieri_factorizations().cardinality()
                8

                sage: W.from_reduced_word([1,3]).left_pieri_factorizations().cardinality()
                4
                sage: W.from_reduced_word([1,3,4,3]).left_pieri_factorizations().cardinality()
                8

                sage: W.from_reduced_word([2,1]).left_pieri_factorizations().cardinality()
                3
                sage: W.from_reduced_word([1,2]).left_pieri_factorizations().cardinality()
                2
                sage: [W.from_reduced_word([1,2]).left_pieri_factorizations(max_length=i).cardinality() for i in [-1, 0, 1, 2]]
                [0, 1, 2, 2]

                sage: W = WeylGroup(['C',4,1])
                sage: w = W.from_reduced_word([0,3,2,1,0])
                sage: w.left_pieri_factorizations().cardinality()
                7
                sage: [(u.reduced_word(),v.reduced_word()) for (u,v) in w.left_pieri_factorizations()]
                [([], [3, 2, 0, 1, 0]),
                ([0], [3, 2, 1, 0]),
                ([3], [2, 0, 1, 0]),
                ([3, 0], [2, 1, 0]),
                ([3, 2], [0, 1, 0]),
                ([3, 2, 0], [1, 0]),
                ([3, 2, 0, 1], [0])]

                sage: W = WeylGroup(['B',4,1])
                sage: W.from_reduced_word([0,2,1,0]).left_pieri_factorizations().cardinality()
                6
            """
            if max_length is None:
                from sage.rings.infinity import infinity
                max_length = infinity
            pieri_factors = self.parent().pieri_factors()
            def predicate(u):
                return u in pieri_factors and u.length() <= max_length

            return self.binary_factorizations(predicate)

        @cached_in_parent_method
        def stanley_symmetric_function_as_polynomial(self, max_length=None):
            r"""
            Returns a multivariate generating function for the number
            of factorizations of a Weyl group element into Pieri
            factors of decreasing length, weighted by a statistic on
            Pieri factors.

            .. SEEALSO::

                * :meth:`stanley_symmetric_function`
                * :meth:`WeylGroups.ParentMethods.pieri_factors`
                * :mod:`sage.combinat.root_system.pieri_factors`

            INPUT:

                - ``self`` -- an element `w` of a Weyl group `W`
                - ``max_length`` -- a non negative integer or infinity (default: infinity)

            Returns the generating series for the Pieri factorizations
            `w = u_1 \cdots u_k`, where `u_i` is a Pieri factor for
            all `i`, `l(w) = \sum_{i=1}^k l(u_i)` and
            ``max_length`` `\geq l(u_1) \geq \cdots \geq l(u_k)`.

            A factorization `u_1 \cdots u_k` contributes a monomial of
            the form `\prod_i x_{l(u_i)}`, with coefficient given by
            `\prod_i 2^{c(u_i)}`, where `c` is a type-dependent
            statistic on Pieri factors, as returned by the method
            ``u[i].stanley_symm_poly_weight()``.

            EXAMPLES::

                sage: W = WeylGroup(['A', 3, 1])
                sage: W.from_reduced_word([]).stanley_symmetric_function_as_polynomial()
                1
                sage: W.from_reduced_word([1]).stanley_symmetric_function_as_polynomial()
                x1
                sage: W.from_reduced_word([1,2]).stanley_symmetric_function_as_polynomial()
                x1^2
                sage: W.from_reduced_word([2,1]).stanley_symmetric_function_as_polynomial()
                x1^2 + x2
                sage: W.from_reduced_word([1,2,1]).stanley_symmetric_function_as_polynomial()
                2*x1^3 + x1*x2
                sage: W.from_reduced_word([1,2,1,0]).stanley_symmetric_function_as_polynomial()
                3*x1^4 + 2*x1^2*x2 + x2^2 + x1*x3
                sage: W.from_reduced_word([1,2,3,1,2,1,0]).stanley_symmetric_function_as_polynomial() # long time
                22*x1^7 + 11*x1^5*x2 + 5*x1^3*x2^2 + 3*x1^4*x3 + 2*x1*x2^3 + x1^2*x2*x3
                sage: W.from_reduced_word([3,1,2,0,3,1,0]).stanley_symmetric_function_as_polynomial() # long time
                8*x1^7 + 4*x1^5*x2 + 2*x1^3*x2^2 + x1*x2^3

                sage: W = WeylGroup(['C',3,1])
                sage: W.from_reduced_word([0,2,1,0]).stanley_symmetric_function_as_polynomial()
                32*x1^4 + 16*x1^2*x2 + 8*x2^2 + 4*x1*x3

                sage: W = WeylGroup(['B',3,1])
                sage: W.from_reduced_word([3,2,1]).stanley_symmetric_function_as_polynomial()
                2*x1^3 + x1*x2 + 1/2*x3

            Algorithm: Induction on the left Pieri factors. Note that
            this induction preserves subsets of `W` which are stable
            by taking right factors, and in particular Grassmanian
            elements.
            """
            if max_length is None:
                from sage.rings.infinity import infinity
                max_length = infinity
            W = self.parent()
            pieri_factors = W.pieri_factors()
            from sage.rings.rational_field import QQ
            R = QQ[','.join('x%s'%l for l in range(1,pieri_factors.max_length()+1))]
            x = R.gens()
            if self.is_one():
                return R.one()

            return R(sum(2**(pieri_factors.stanley_symm_poly_weight(u))*x[u.length()-1] * v.stanley_symmetric_function_as_polynomial(max_length = u.length())
                           for (u,v) in self.left_pieri_factorizations(max_length)
                           if u != W.one()))


        def stanley_symmetric_function(self):
            r"""
            Return the affine Stanley symmetric function indexed by ``self``.

            INPUT:

            - ``self`` -- an element `w` of a Weyl group

            Returns the affine Stanley symmetric function indexed by
            `w`. Stanley symmetric functions are defined as generating
            series of the factorizations of `w` into Pieri factors and
            weighted by a statistic on Pieri factors.

            .. SEEALSO::

                * :meth:`stanley_symmetric_function_as_polynomial`
                * :meth:`WeylGroups.ParentMethods.pieri_factors`
                * :mod:`sage.combinat.root_system.pieri_factors`

            EXAMPLES::

                sage: W = WeylGroup(['A', 3, 1])
                sage: W.from_reduced_word([3,1,2,0,3,1,0]).stanley_symmetric_function()
                8*m[1, 1, 1, 1, 1, 1, 1] + 4*m[2, 1, 1, 1, 1, 1] + 2*m[2, 2, 1, 1, 1] + m[2, 2, 2, 1]
                sage: A = AffinePermutationGroup(['A',3,1])
                sage: A.from_reduced_word([3,1,2,0,3,1,0]).stanley_symmetric_function()
                8*m[1, 1, 1, 1, 1, 1, 1] + 4*m[2, 1, 1, 1, 1, 1] + 2*m[2, 2, 1, 1, 1] + m[2, 2, 2, 1]

                sage: W = WeylGroup(['C',3,1])
                sage: W.from_reduced_word([0,2,1,0]).stanley_symmetric_function()
                32*m[1, 1, 1, 1] + 16*m[2, 1, 1] + 8*m[2, 2] + 4*m[3, 1]

                sage: W = WeylGroup(['B',3,1])
                sage: W.from_reduced_word([3,2,1]).stanley_symmetric_function()
                2*m[1, 1, 1] + m[2, 1] + 1/2*m[3]

                sage: W = WeylGroup(['B',4])
                sage: w = W.from_reduced_word([3,2,3,1])
                sage: w.stanley_symmetric_function()  # long time (6s on sage.math, 2011)
                48*m[1, 1, 1, 1] + 24*m[2, 1, 1] + 12*m[2, 2] + 8*m[3, 1] + 2*m[4]

                sage: A = AffinePermutationGroup(['A',4,1])
                sage: a = A([-2,0,1,4,12])
                sage: a.stanley_symmetric_function()
                6*m[1, 1, 1, 1, 1, 1, 1, 1] + 5*m[2, 1, 1, 1, 1, 1, 1] + 4*m[2, 2, 1, 1, 1, 1]
                + 3*m[2, 2, 2, 1, 1] + 2*m[2, 2, 2, 2] + 4*m[3, 1, 1, 1, 1, 1] + 3*m[3, 2, 1, 1, 1]
                + 2*m[3, 2, 2, 1] + 2*m[3, 3, 1, 1] + m[3, 3, 2] + 3*m[4, 1, 1, 1, 1] + 2*m[4, 2, 1, 1]
                + m[4, 2, 2] + m[4, 3, 1]

            One more example (:trac:`14095`)::

                sage: G = SymmetricGroup(4)
                sage: w = G.from_reduced_word([3,2,3,1])
                sage: w.stanley_symmetric_function()
                3*m[1, 1, 1, 1] + 2*m[2, 1, 1] + m[2, 2] + m[3, 1]

            REFERENCES:

            - [BH1994]_

            - [Lam2008]_

            - [LSS2009]_

            - [Pon2010]_
            """
            import sage.combinat.sf
            from sage.rings.rational_field import QQ
            m = sage.combinat.sf.sf.SymmetricFunctions(QQ).monomial()
            return m.from_polynomial_exp(self.stanley_symmetric_function_as_polynomial())

        @cached_in_parent_method
        def reflection_to_root(self):
            r"""
            Returns the root associated with the reflection ``self``.

            EXAMPLES::

                sage: W=WeylGroup(['C',2],prefix="s")
                sage: W.from_reduced_word([1,2,1]).reflection_to_root()
                2*alpha[1] + alpha[2]
                sage: W.from_reduced_word([1,2]).reflection_to_root()
                Traceback (most recent call last):
                ...
                ValueError: s1*s2 is not a reflection
                sage: W.long_element().reflection_to_root()
                Traceback (most recent call last):
                ...
                ValueError: s2*s1*s2*s1 is not a reflection
            """

            i = self.first_descent()
            if i is None:
                raise ValueError("{} is not a reflection".format(self))
            if self == self.parent().simple_reflection(i):
                return self.parent().cartan_type().root_system().root_lattice().simple_root(i)
            rsi = self.apply_simple_reflection(i)
            if not rsi.has_descent(i, side='left'):
                raise ValueError("{} is not a reflection".format(self))
            return rsi.apply_simple_reflection(i, side='left').reflection_to_root().simple_reflection(i)

        @cached_in_parent_method
        def reflection_to_coroot(self):
            r"""
            Returns the coroot associated with the reflection ``self``.

            EXAMPLES::

                sage: W=WeylGroup(['C',2],prefix="s")
                sage: W.from_reduced_word([1,2,1]).reflection_to_coroot()
                alphacheck[1] + alphacheck[2]
                sage: W.from_reduced_word([1,2]).reflection_to_coroot()
                Traceback (most recent call last):
                ...
                ValueError: s1*s2 is not a reflection
                sage: W.long_element().reflection_to_coroot()
                Traceback (most recent call last):
                ...
                ValueError: s2*s1*s2*s1 is not a reflection
            """

            i = self.first_descent()
            if i is None:
                raise ValueError("{} is not a reflection".format(self))
            if self == self.parent().simple_reflection(i):
                return self.parent().cartan_type().root_system().root_lattice().simple_coroot(i)
            rsi = self.apply_simple_reflection(i)
            if not rsi.has_descent(i, side='left'):
                raise ValueError("{} is not a reflection".format(self))
            return rsi.apply_simple_reflection(i, side='left').reflection_to_coroot().simple_reflection(i)

        def inversions(self, side = 'right', inversion_type = 'reflections'):
            """
            Returns the set of inversions of ``self``.

            INPUT:

            - ``side`` -- 'right' (default) or 'left'
            - ``inversion_type`` -- 'reflections' (default), 'roots', or 'coroots'.

            OUTPUT:

            For reflections, the set of reflections r in the Weyl group such that
            ``self`` ``r`` < ``self``. For (co)roots, the set of positive (co)roots that are sent
            by ``self`` to negative (co)roots; their associated reflections are described above.

            If ``side`` is 'left', the inverse Weyl group element is used.

            EXAMPLES::

                sage: W=WeylGroup(['C',2], prefix="s")
                sage: w=W.from_reduced_word([1,2])
                sage: w.inversions()
                [s2, s2*s1*s2]
                sage: w.inversions(inversion_type = 'reflections')
                [s2, s2*s1*s2]
                sage: w.inversions(inversion_type = 'roots')
                [alpha[2], alpha[1] + alpha[2]]
                sage: w.inversions(inversion_type = 'coroots')
                [alphacheck[2], alphacheck[1] + 2*alphacheck[2]]
                sage: w.inversions(side = 'left')
                [s1, s1*s2*s1]
                sage: w.inversions(side = 'left', inversion_type = 'roots')
                [alpha[1], 2*alpha[1] + alpha[2]]
                sage: w.inversions(side = 'left', inversion_type = 'coroots')
                [alphacheck[1], alphacheck[1] + alphacheck[2]]

            """

            if side == 'left':
                self = self.inverse()
            reflections = self.inversions_as_reflections()
            if inversion_type == 'reflections':
                return reflections
            if inversion_type == 'roots':
                return [r.reflection_to_root() for r in reflections]
            if inversion_type == 'coroots':
                return [r.reflection_to_coroot() for r in reflections]
            raise ValueError("inversion_type {} is invalid".format(inversion_type))

        def inversion_arrangement(self, side='right'):
            r"""
            Return the inversion hyperplane arrangement of ``self``.

            INPUT:

            - ``side`` -- ``'right'`` (default) or ``'left'``

            OUTPUT:

            A (central) hyperplane arrangement whose hyperplanes correspond
            to the inversions of ``self`` given as roots.

            The ``side`` parameter determines on which side
            to compute the inversions.

            EXAMPLES::

                sage: W = WeylGroup(['A',3])
                sage: w = W.from_reduced_word([1, 2, 3, 1, 2])
                sage: A = w.inversion_arrangement(); A
                Arrangement of 5 hyperplanes of dimension 3 and rank 3
                sage: A.hyperplanes()
                (Hyperplane 0*a1 + 0*a2 + a3 + 0,
                 Hyperplane 0*a1 + a2 + 0*a3 + 0,
                 Hyperplane 0*a1 + a2 + a3 + 0,
                 Hyperplane a1 + a2 + 0*a3 + 0,
                 Hyperplane a1 + a2 + a3 + 0)

            The identity element gives the empty arrangement::

                sage: W = WeylGroup(['A',3])
                sage: W.one().inversion_arrangement()
                Empty hyperplane arrangement of dimension 3
            """
            inv = self.inversions(side=side, inversion_type='roots')
            from sage.geometry.hyperplane_arrangement.arrangement import HyperplaneArrangements
            I = self.parent().cartan_type().index_set()
            from sage.rings.rational_field import QQ
            H = HyperplaneArrangements(QQ, tuple(['a{}'.format(i) for i in I]))
            gens = H.gens()
            if not inv:
                return H()
            return H([sum(c * gens[I.index(i)] for (i, c) in root)
                      for root in inv])

        def bruhat_lower_covers_coroots(self):
            r"""
            Return all 2-tuples (``v``, `\alpha`) where ``v`` is covered
            by ``self`` and `\alpha` is the positive coroot such that
            ``self`` = ``v`` `s_\alpha` where `s_\alpha` is
            the reflection orthogonal to `\alpha`.

            ALGORITHM:

            See :meth:`.bruhat_lower_covers` and
            :meth:`.bruhat_lower_covers_reflections` for Coxeter groups.

            EXAMPLES::

                sage: W = WeylGroup(['A',3], prefix="s")
                sage: w = W.from_reduced_word([3,1,2,1])
                sage: w.bruhat_lower_covers_coroots()
                [(s1*s2*s1, alphacheck[1] + alphacheck[2] + alphacheck[3]),
                 (s3*s2*s1, alphacheck[2]), (s3*s1*s2, alphacheck[1])]
            """
            return [(x[0],x[1].reflection_to_coroot())
                    for x in self.bruhat_lower_covers_reflections()]

        def bruhat_upper_covers_coroots(self):
            r"""
            Returns all 2-tuples (``v``, `\alpha`) where ``v`` is covers ``self`` and `\alpha`
            is the positive coroot such that ``self`` = ``v`` `s_\alpha` where `s_\alpha` is
            the reflection orthogonal to `\alpha`.

            ALGORITHM:

            See :meth:`~CoxeterGroups.ElementMethods.bruhat_upper_covers` and :meth:`.bruhat_upper_covers_reflections` for Coxeter groups.

            EXAMPLES::

                sage: W = WeylGroup(['A',4], prefix="s")
                sage: w = W.from_reduced_word([3,1,2,1])
                sage: w.bruhat_upper_covers_coroots()
                [(s1*s2*s3*s2*s1, alphacheck[3]),
                 (s2*s3*s1*s2*s1, alphacheck[2] + alphacheck[3]),
                 (s3*s4*s1*s2*s1, alphacheck[4]),
                 (s4*s3*s1*s2*s1, alphacheck[1] + alphacheck[2] + alphacheck[3] + alphacheck[4])]
            """
            return [(x[0],x[1].reflection_to_coroot())
                    for x in self.bruhat_upper_covers_reflections()]

        def quantum_bruhat_successors(self, index_set=None, roots=False, quantum_only=False):
            r"""
            Return the successors of ``self`` in the quantum Bruhat graph
            on the parabolic quotient of the Weyl group determined by the
            subset of Dynkin nodes ``index_set``.

            INPUT:

            - ``self`` -- a Weyl group element, which is assumed to
              be of minimum length in its coset with respect to the
              parabolic subgroup

            - ``index_set`` -- (default: ``None``) indicates the set of
              simple reflections used to generate the parabolic subgroup;
              the default value indicates that the subgroup is the identity

            - ``roots`` -- (default: ``False``) if ``True``, returns the
              list of 2-tuples (``w``, `\alpha`) where ``w`` is a successor
              and `\alpha` is the positive root associated with the
              successor relation

            - ``quantum_only`` -- (default: ``False``) if ``True``, returns
              only the quantum successors

            EXAMPLES::

                sage: W = WeylGroup(['A',3], prefix="s")
                sage: w = W.from_reduced_word([3,1,2])
                sage: w.quantum_bruhat_successors([1], roots = True)
                [(s3, alpha[2]), (s1*s2*s3*s2, alpha[3]),
                 (s2*s3*s1*s2, alpha[1] + alpha[2] + alpha[3])]
                sage: w.quantum_bruhat_successors([1,3])
                [1, s2*s3*s1*s2]
                sage: w.quantum_bruhat_successors(roots = True)
                [(s3*s1*s2*s1, alpha[1]),
                 (s3*s1, alpha[2]),
                 (s1*s2*s3*s2, alpha[3]),
                 (s2*s3*s1*s2, alpha[1] + alpha[2] + alpha[3])]
                sage: w.quantum_bruhat_successors()
                [s3*s1*s2*s1, s3*s1, s1*s2*s3*s2, s2*s3*s1*s2]
                sage: w.quantum_bruhat_successors(quantum_only = True)
                [s3*s1]
                sage: w = W.from_reduced_word([2,3])
                sage: w.quantum_bruhat_successors([1,3])
                Traceback (most recent call last):
                ...
                ValueError: s2*s3 is not of minimum length in its coset of the parabolic subgroup generated by the reflections (1, 3)
            """
            W = self.parent()
            if not W.cartan_type().is_finite():
                raise ValueError("the Cartan type {} is not finite".format(W.cartan_type()))
            if index_set is None:
                index_set = []
            else:
                index_set = [x for x in index_set]
            index_set = tuple(index_set)
            if self != self.coset_representative(index_set):
                raise ValueError("{} is not of minimum length in its coset of the parabolic subgroup generated by the reflections {}".format(self, index_set))
            lattice = W.cartan_type().root_system().root_lattice()
            w_length_plus_one = self.length() + 1
            successors = []
            for alpha in lattice.nonparabolic_positive_roots(index_set):
                wr = self * W.from_reduced_word(alpha.associated_reflection())
                wrc = wr.coset_representative(index_set)
                if wrc == wr and wr.length() == w_length_plus_one and not quantum_only:
                    if roots:
                        successors.append((wr,alpha))
                    else:
                        successors.append(wr)
                elif alpha.quantum_root() and wrc.length() == w_length_plus_one - lattice.nonparabolic_positive_root_sum(index_set).scalar(alpha.associated_coroot()):
                    if roots:
                        successors.append((wrc,alpha))
                    else:
                        successors.append(wrc)
            return successors

